leveldb: fix static build

leveldb: fix static build

Since version 1.19 Makefile variable names changed. Update the build command
accordingly.

While at it, remove variables that Makefile doesn't use anymore. This change
means that the static library is always build, even when BR2_SHARED_LIBS=y,
since the static library name is hard coded in leveldb Makefile.

Fixes:
http://autobuild.buildroot.net/results/00c/00c8c5a1d6001eaebfe2a9f848dd292c91bf6a79/

Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>